In the operation of knitting machines, needles having hooks at one end are moved in reciprocating fashion to engage yarns and to pull them into loops or various structures so as to form a knitted fabric. In general, knitting machines may be classified into two types: flat bed knitting machines and circular knitting machines. In both types of machines, means is provided for urging the needles into reciprocating motion. For example, in many knitting machines the needles are provided with one or more butts that extend from the side of a medial portion of the needle shaft. This butt fits into a cam that has upper and lower faces defining a generally undulating cam track. The cam moves laterally relative to the needles, thus urging the butts to follow the cam track and thereby causing the needles to move in translation due to displacement of the cam track.
As the needles move in translation, the hooks on the ends thereof engage yams and manipulate the yams to form a knitted fabric. If the hook on the end of a latch needle is broken or if it fails to engage a yarn, the latch needle can no longer perform the function of forming a loop. If a broken hook is undetected, the circular knitting machine will continue to operate, but the broken hook will cause a continuing defect in the knitted fabric. These defects are unacceptable in the knitted product and therefore result in loss of salable knitted fabric. It is therefore desirable to provide a system for detecting when a needle hook is broken or otherwise nonfunctional and to stop operation of the knitting machine soon after the broken hook is detected so that the broken needle may be replaced.
Apparatus and methods for detecting and reducing fabric defects for use with circular knitting machines are known. For example, U.S. Pat. No. 3,785,177 (the "Hino patent") shows a device for sensing a broken yarn of a circular knitting machine. As an initial matter, the invention in the Hino patent is addressed to broken yarns, and not to broken hooks. Detecting broken yarns is particularly important in knitting machines for producing underwear and undergarments. In the Hino patent, the sensor and associated cam structure are positioned at the base of a stitch cam. Importantly, positioning of the sensor at the base of a stitch cam requires some spacing in the cam for the free deflection of needles with broken hooks or yarns. Placing the sensor at the bottom of the stitch cam, with the corresponding free space required, is impractical in modern high-speed knitting machines, which require very accurate and precise control of the needle butt during the stitch-forming process.
Other references show various apparatus and methods for detecting broken hooks in latch needles in knitting machines. Many of the references focus their detecting devices at or near the hook portion of the needle rather than at the butt portion of the needle. For example, U.S. Pat. No. 3,659,437, MacArthur et al., shows a defective needle detector that uses a beam of collimated energy directed to the hook end of the needles to detect the presence or absence of hooks. Alternatively, a magnet with a piece of electrical crystal may be mounted in the proximity of the needle hooks.
U.S. Pat. No. 3,904,529, Nakamura, shows an apparatus for detecting defective needles which uses an optical fiber having an end disposed so as to detect light reflections from hooks or latch portions of needles in a similar manner to MacArthur et al.
U.S. Pat. No. 3,905,211, Raisin, et al., shows another detector apparatus using magnetic detectors on a hosiery loom. This system employs a magnetic detector mounted to the outside of the loom, and the detector analyzes the entire needle rather than focusing on the needle hook.
The above-described attempts to detect broken needles and to stop operation of the machine can be characterized as failing to provide for high speed operation while simultaneously maintaining precise accurate control of needles as they pass through the stitch cam. In contrast, U.S. Pat. No. 6,035,669 to Alan Gutschmit (the "Gutschmit patent") and owned by Monarch Knitting Machinery Corp. discloses an embodiment for detecting broken hooks of needles in a knitting machine while providing for high speed operation and maintaining precise accurate control of the needles as they pass through the stitch cam, as described in the remainder of this paragraph. First and second cam faces define a cam for slidably receiving needle butts. The cam includes a raising cam portion, a stitch cam portion, and a gate cam portion (e.g., welt-cam portion) in which tension forces between yam loops and hooks bias the butts of intact needles against the first cam face. The detector has a detector butt raising segment wherein the second cam face in the gate cam portion is inclined away from the second cam face at the stitch position for urging butts toward the first cam face. A detector butt lowering segment follows the detector butt raising segment, in which the first cam face urges the butts toward the second cam face. A recess segment follows the butt lowering segment. The recess is formed so that the second cam face is sufficiently spaced from the first cam face so that butts of needles having broken hooks are urged into the recess segment by the detector butt lowering segment, but butts of intact needles that are biased against the first cam face do not enter the detector recess segment. A sensor identifies butts of broken needles in the detector recess segment. The apparatus may further include means for stopping operation of the knitting machine when the sensor is activated. A method for detecting broken hooks is also provided. The Gutschmit patent is incorporated herein by reference.
Whereas the specific embodiment described in the Gutschmit patent provides great advances in that it can detect a broken needle and stop operation of a knitting machine while facilitating high speed operation and simultaneously maintaining precise accurate control of needles as they pass through the stitch cam, it would require a sensor for each cam of a knitting machine having multiple cams for respectively causing the needles to reciprocate. In knitting machines having multiple cams for causing the needles to reciprocate, the butts of different needles are received in and driven by the different cam tracks. It can be disadvantageous to have to mount, align and maintain multiple sensors for detecting broken needles in a machine having multiple cams for respectively causing the needles to reciprocate. Accordingly, there is a need in the art for methods and apparatus that, in addition to providing for high speed operation while simultaneously maintaining precise accurate control of needles as they pass through the stitch cam, can provide for the efficient detection of broken hooks in a knitting machine having multiple cams for causing the needles to reciprocate. Likewise there is a need in the art for needles for use with the needed methods and apparatus.